I've never had a Cottonmouth fall out of a tree, but I had a Northern Black Racer fall onto my kayak as I paddled under brush. Scared the [bleep] out of me. When it comes to shear meanness, Black Racers are hard to beat. It's the only snake that has literally come at me and tried to attack when I was near. I stopped picking them up because they are so pissy. And their musk stinks worse than any other snake I've handed except maybe a Smooth Green Snake.
Here's one of the videos I took of one of the little Cottons that I found, trying to fool me into believing he's a rattler.
https://photos.app.goo.gl/1YFw8syGadrwcLB69
